One major drawback of highly active simple zirconocene dihalide systems such as bis(n-butylcyclopentadienyl)zirconium dichloride for ethylene copolymerization has been the premature chain termination through beta-hydride elimination of the inserted alpha-olefin. As a result, high molecular weight ethylene-alpha olefin copolymers can not be produced by this group of catalysts under commercial operation conditions.
A typical solution for this deficiency has been the replacement of simple cyclopentadienyl groups with structurally more elaborate ligands. The synthesis of such ligands are, in general, time-consuming and laborious; and the resulting catalysts are, very often, less active.
U.S. Pat. No. 4,931,517 assigned to Mitsubishi Petrochemical discloses the use of silicon compounds having an Si--O--C bond as a molecular weight regulator for polyethylene produced using by the combination of certain metallocene and an aluminoxane. However, to be effective in boosting the molecular weight, a Si/Zr ratio of 40 or higher was employed. But with these Si/Zr ratios, the metallocene catalysts employed suffered greater than 50% loss in catalyst activity.
WO 93/13140 assigned to Exxon discloses the use of a phenol component of a catalyst system along with a monocyclopentadienyl Group IVB transition metal compound and aluminoxane. However, the ethylene polymers produced by this catalyst system displayed lower molecular weight.
Indemitsu Kosan's Japanese Application No. H7-160441 (Disclosure No. H8-231622) discloses a catalyst containing a monocyclopentadienyl compound, aluminoxane, and a phenolic compound. However, the phenolic compounds employed are not capable of forming chelatable groups on the 2 and 6 positions, since it is known to lead to deactivation or to show no catalytic activity with a monocyclopentadienyl compound. In this catalyst, the 2,6-substituent is an alkyl group.
EP 0 630 910 assigned to Union Carbide discloses a method for reversibly controlling the activity of an olefin polymerization catalyst by the addition of a Lewis base containing an available pair of electrons capable of forming a dative bond to both the metallocene and the aluminoxane. Here Lewis bases are employed for activity reduction and/or to control static.
It would be desirable to develop some simple modifications that would enable a simple zirconocene system to produce ethylene copolymers with high molecular weights and to maintain high catalytic activity.